System of coils for producing magnetic field gradients of very uniform polarization in an imaging or spectroscopy installation by NMR

Abstract

A system of coils is disclosed for the production of imaging by nuclear magnetic resonance of gradients Gx and Gy. The system of coils is disposed on the outer surface of a straight cylinder of revolution of axis Oz and radius R. The coils include four patterns of conductor wires which are pairwise symmetrical with respect to a center O and pairwise symmetrical with respect to the vertical plane xOy. Each pattern has three arcs participating in the production of the gradient which is required. That is, two outward arcs are located close to the center O at respective distances h 1 and h 2 from plane xOy and a return arc is spaced further from the center O and is located at a distance H 3 from the plane xOy. Each arc has three section a, b and c, with the two end sections a and c having the same length being traversed by a current which has one-half the intensity of the current passing through the central section b.

What is claimed is: 
     
       1. A system of coils for producing in imaging, spectroscopy or spectroscopic imaging by nuclear magnetic resonance, gradients G x , G y  of magnetic polarization fields which are very uniform in a direction (Ox or Oy) perpendicular to a magnetic polarization field produced in direction Oz, whereby said system of coils is placed on and fitted to the outer surface of a straight cylinder of revolution of axis Oz and radius R, wherein said system of coils comprises four patterns of conductor wires pairwise symmetrical with respect to a centre O and pairwise symmetrical with respect to the vertical plane xoy, each pattern having current lines or arcs disposed in accordance with circular section portions of the straight cylinder and distributed in the following manner; each pattern having three arcs participating in the production of the sought gradient, wherein two "outward" arcs are located close to centre O at respective distances h 1  and h 2  from plane xOy and one "return" arc spaced from centre O and located at a distance h 3  from plane xOy where h 3  is greater than h 1  or h 2  ;   and wherein each arc has three sections a, b and c, whereof the two end sections a and c are of equal length, the "outward" currents in the first section of the first two arcs being respectively equal to I and J and the "return" current in the first section of the third arc being equal to I+J; wherein the ratios h 1  /R, h 2  /r and h 3  /R are substantially as shown in the following table: ##STR2##   
     
     
       2. A system of coils according to claim 1, wherein the number of turns N1 and N2 in the "outward" arcs and N3 in the "return" arc of each pattern are chosen in the following table in which, for each group of values, there is N3=-(N1+N2):   ______________________________________                                    
N1              N2          N3                                            
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1               4           -5                                            
1               3           -4                                            
3               8           -11                                           
2               5           -7                                            
3               7           -10                                           
1               2           -3                                            
4               7           -11                                           
3               5           -8                                            
2               3           -5                                            
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       3. A system of coils according to any one of claims 1 or 2, further including angles alpha1, alpha2, alpha3 and alpha4 representing the spacing, from axis Oz, of the sides of each pattern parallel to said axis, said angles having values which are substantially as shown in the following table:   ______________________________________                                    
alpha 1    alpha 2    alpha 3    alpha 4                                  
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22-24      82-84       96-98     154-156                                  
12-14      48-50      130-132    166-168                                  
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       4. A system of coils according to claim 1, wherein the central section b of each arc comprises two identical coils angularly displaced by rotation about axis Oz symmetrically with respect to plane xOz and traversed in series by the same current. 
     
     
       5. A system of coils according to claim 1, wherein the central section b of each path is comprises two independent imbricated coils of different widths, having the plane xOz as the plane of symmetry and traversed by equal currents. 
     
     
       6. A system of coils according to either of the claims 1 or 2, wherein the length of central section b of each arc is reduced to zero and further including angles alpha1 and alpha2 representing the spacing, from axis Oz, of the sides of each pattern parallel to said axis, said angles having values which are substantially as shown in the following table:   ______________________________________                                    
       alpha 1       alpha 2                                              
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       26-30         150-154                                              
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       7. A system of coils according to claim 1, wherein the four patterns thereof are traversed by the same intensity in series and the desired intensities I and J at each point of the circuit are obtained by the choice of the number of respective turns thereof N1, N2 and N3. 
     
     
       8. A system of coils according to claim 1 wherein said end sections a and c are traversed by a current having an intensity half that of the current passing through said central section b.